The invention relates to a process for the removal of conjugated olefins from a monoolefin stream such as a stream containing normal alpha olefins.
Monoolefins such as normal alpha olefins can be obtained from streams that have been subjected to, for example, dehydrogenation, cracking, or ethylene oligomerization. Depending upon the production method, the normal alpha olefin stream can contain varying amounts of conjugated olefins. If present in large amounts, the conjugated olefins may be separated from the monoolefins for sale or other use. Methods useful for such a separation include distillation, selective adsorption, selective hydrogenation of the conjugated olefin, dimerization of the conjugated olefin, or complexation of the conjugated olefin.
However, it is very difficult to reduce the conjugated olefins content in a normal alpha olefin stream to low levels by means such as distillation because some isomers have very close boiling points and may form azeotropes with one another preventing complete separation. It is particularly difficult to reduce the levels of conjugated olefins in monoolefin streams below a few hundred parts per million (ppm) by the methods mentioned above. Conjugated olefins can be very undesirable impurities in monoolefins such as normal alpha olefins even in very low concentrations such as a few hundred ppm. Normal alpha olefins (NAOs) are used for applications such as polymerization of monomers to form polyolefins. Examples include polymerization of ethylene to form polyethylene and polymerization of propylene to form polypropylene. Normal alpha olefins such as 1-butene, 1-hexene, and 1-octene, are used in the polyethylene process to provide branching of the resultant polymer. Any conjugated olefin present in a stream containing normal alpha olefins can have a very undesirable impact on the production of polyolefins through catalyst deactivation thus reducing catalyst productivity or by causing crosslinking of the polymer.
As mentioned above, one method currently used to remove conjugated olefins from olefin streams is by selective hydrogenation. Conjugated olefins can be selectively hydrogenated under the proper conditions and using an appropriate catalyst. Selective hydrogenation allows one to greatly reduce the levels of conjugated olefins in the olefin stream. One commercial process that uses selective hydrogenation is UOP""s DeFine process. Selective hydrogenation has the disadvantage in that it is difficult to selectively hydrogenate all of the conjugated olefin without hydrogenating significant amounts of monoolefin or isomerizing a normal alpha olefin to an internal olefin. On the other hand, if one minimizes hydrogenation of the monoolefins significant amounts of conjugated olefins are left unconverted.
Dimerization of the conjugated olefin is also a possible way to remove such conjugated olefin from a monoolefin stream. However, this method does not reduce the conjugated olefin content to very low levels.
For the reasons discussed above, it would be very desirable to have an efficient and economical separation/purification process for the removal of even very low levels of conjugated olefins from a monoolefin stream. The present invention provides such a process.
The present invention relates to a process for the removal of conjugated olefins from a monoolefin-containing fluid. The process of the present invention employs the use of a Diels-Alder dienophile to selectively react with the conjugated olefins to form a Diels-Alder adduct. The Diels-Alder adduct can then be separated or removed from the monoolefin-containing fluid by conventional separation means such as distillation, adsorption, membrane separation and the like and combinations thereof. Another means to accomplish the separation or removal of conjugated olefins from a monoolefin-containing fluid is through the application of the invention using a Diels-Alder dienophile in a reactive distillation.
The present invention comprises a process for removing conjugated olefins from a fluid containing such conjugated olefins and monoolefins to provide a monoolefin-containing product depleted in conjugated olefins, comprising:
(a) contacting a fluid comprising conjugated olefins and monoolefins with a Diels-Alder dienophile to provide a fluid comprising a Diels-Alder adduct and monoolefins;
(b) separating the monoolefins from the fluid comprising a Diels-Alder adduct and monoolefins of (a); and
(c) recovering a resulting monoolefin-containing fluid depleted in conjugated olefins.
Alternatively, the process of the present invention can be characterized as a process for the separation of conjugated olefins and monoolefins, comprising:
contacting a fluid comprising monoolefins and conjugated olefins with a Diels-Alder dienophile to provide a fluid comprising a Diels-Alder adduct and monoolefins; and
recovering a resulting monoolefin-containing fluid comprising less than about 50 ppm of conjugated olefins.
The present invention provides a method to efficiently reduce the levels of conjugated olefin-contaminant or contaminants in a resulting monoolefin-containing fluid to very low levels without substantial losses of the monoolefin and without introducing other contaminants. Surprisingly the present invention is able to effectively reduce the conjugated diene levels of the resulting monoolefin-containing product to less than about 50 ppm. The process of the present invention utilizes a minimum of new equipment and can utilize existing separation methods to provide the required separation of the adduct from the mixture comprising such adduct and monoolefins.